This invention relates to the sensing of gases dissolved in liquids and is specifically adapted for the in vivo sensing of blood gases. Blood gases have been sensed and analyzed by various prior art methods, one of which being disclosed in U.S. Pat. Nos. 3,983,864 and 4,016,864. As shown in those patents, a carrier gas is introduced into a special catheter probe, and held in a chamber where the blood gases equilibrate through a gas permeable membrane with the carrier gas and the carrier gas containing the equilibrated blood gases is thereafter withdrawn and analyzed.
The catheter is introduced in vivo into the particular blood vessel sought to be analyzed. An equilibration chamber is provided in the probe and allows an equilibration between the carrier gas passing through the probe and the blood gases contained in the blood. Equilibration occurs through a gas permeable membrane that surrounds the equilibration chamber and has its outside surface in direct contact with the blood to be analyzed. The blood gases pass into the equilibration chamber through the gas permeable material until the partial pressures within the chamber achieve blood levels.
The carrier gas remains in the equilibration chamber for a specific period of time to insure equilibration is completed, at which time, the carrier gas containing that bolus of carrier gas with the equilibrated blood gases is removed and its content determined by an analyzer such as a gas chromatograph.
A vacuum means is used to transport the bolus of equilibrated gases to the analyzer through various valving means.
One of the difficulties with such present systems is that a certain finite time is needed for the sample gas to fully equilibrate with the blood gases and thus, the number of samples one can take within any specific period of time is limited. That time is partially dependent upon the ratio of area of the equilibration chamber that directly receives the blood gases through the permeable material to the volume of the equilibration chamber as well as other factors such as the gas permeability of the membrane. To be effective and rapid, the aforementioned ratio should be high, that is, there needs to be a large surface area through which the blood gases pass into the equilibration chamber in relation to the volume of carrier gas in the chamber.
The equilibration chamber itself is normally located at the distal end of the catheter and it comprises an active length of the catheter at that distal end. Present catheters have a relatively long active length in order to include a sufficiently large equilibration chamber and therefore a technician utilizing the catheter may be unable to pinpoint the exact location in the blood vessel where the blood gases are being analyzed. It is therefore desirable that the active length of the catheter be minimized.
A further disadvantage of prior art catheters is in the carrier gas passing through the catheter through dissimilar pathway areas, that is, an internal mixing takes place when the pathway of the gas through the catheter passes from one cross-sectional area to another that is significantly dissimilar in size. A mixing occurs between the equilibrated bolus and its edges that are surrounded by the carrier gas so that the defined edges of the bolus itself are disrupted.